Hello again, I have decided to run the simulation a bit more to make the convergence a bit more perfect. I can’t get my head around the y plus value. Is there a general rule of thumb I can use for the y plus value?

So the y+ is 30 to 100 in the finest spot of the mesh or the first and most refined region, what should be the y+ of the 2nd region refinment and the flow region? I’d don’t understand.

Y+ is only computed for the walls, especially those with wall modeling boundary conditions (function or full resolution). Please refer to this article for some background:

The correct y+ guarantees that the wall boundary condition is giving valid results.

If you are not interested in the accuracy of the flow fields (velocity and pressure) at the walls, but for example in regions far from them, then I guess you do not have to worry about perfect y+ values. Otherwise, you do.